Cheesenonlactate

packaging Multilayer packaging

Glass

SiO2, NaCl, CaCO3, returnable glass

Strong, stiffness Grease resistance

Very low

Extremely low

Jars, coatings (barrier layer)

Aluminum

Aluminum oxides and cryolite

Grease resistance Lightweight

Very low

Extremely low

Foils, lids, cups, tubes

a Subdivisions regarding moisture and oxygen permeability are based on guidelines in Ref. (52). Moisture permeability (gm2-d-1, 38°C, 90% RH): Very low: <1; low: 1-10; medium: 10-50; high: >50. Oxygen permeability (mLm-2d-1atm-1, 20°C, 0% RH): Extremely low: <1; very low: 1-10; low: 10-100; medium: 100-1000; high: >1000.

Cameros goat cheeses, 40-50% CO2 levels (N2 as filler), increased shelf life while maintaining sensory acceptability. 100% CO2 was optimal; from a microbiological point of view, however, the high CO2 levels impaired the sensory quality.

Selection of packaging materials as well as the use of modified atmosphere packaging should be based on the shelf life required. Examples of packaging materials currently used include polystyrene (PS), polypropylene (PP), glass jars, nitrocellulose-coated cellophane, and polyethylene (PE)/polyamide (PA) combinations to obtain oxygen barriers in modified atmosphere packaged products. Lids made of printed aluminum foil with heat-sealable lacquer are often used. Coated aluminum lids are used for products with high salt content, such as Feta cheese, in order to avoid corrosion.

B. Semihard and Hard Cheeses

Matured cheeses are ripened by bacteria. The category includes cheeses such as Edam, Gouda, Danbo, Havarti, Cheddar, Gloucester, Derby, Leicester, Gruy^re, Emmental, Mozzarella, Provolone, and very hard cheeses such as Parmesan and Romano (Italian-type cheeses).

These cheeses require an adequate water vapor barrier as well as low oxygen levels, obtained through the use of high oxygen barrier materials and/or gas packaging. Furthermore, the amount of carbon dioxide should be controlled to avoid unwanted blowing or collapse of the package. Packaging methods include flow packaging, hard and soft deep drawing, vacuum packaging, pillow-pack, stand-up pouches, and shrink-wrapping. Finally, injection-molded units are used for packaging of semihard and hard cheeses. Vacuum packaging is primarily used during ripening and not for consumer packages (34), whereas modified atmosphere packaging is extensively applied for retail packages and bulk products for the catering industry.

The cheeses included in this category may be produced by applying different starter cultures—for example, Cheddar produced from a mesophilic O culture,* Havarti cheese produced from a mesophilic DL starter culture,y and Swiss cheese produced from a thermophilic lactic acid starter and propionic bacteria. These cultures, along with their enzymes and so on, produce and use different gases during their metabolism, which should be taken into consideration when tailoring permeability of the packaging materials and when optimizing the gas composition for modified atmosphere packaging. In Swiss cheeses, such as Emmental cheeses, excessive amounts of CO2 are produced during lactate conversion. Stehle (60) noted that the gas composition in Emmental holes consisted of 95% CO2 and 5% N2. The high CO2 production effectively prevents mold growth, which calls for the use of a highly CO2 permeable film in order to avoid blowing of the package. In contrast, packaging materials used for Cheddar cheese should be sufficiently impermeable to oxygen to prevent oxidation and mold growth, because only minimal amounts of O2 are converted to CO2 in Cheddar. For most materials, the ratio between O2 and CO2 permeability is approximately 1:4 (55). However, recently the use of biobased materials such as polylactic acid-based materials has been suggested for cheeses producing high levels of CO2, because the ratio of O2/CO2 apparently is more favorable for such cheeses (61).

Modified atmosphere packaging is extensively used for consumer-packaged cheeses to increase shelf life. Gas mixes containing 10-40% CO2/60-90% N2 are recommended for retail packages, whereas 30-100% CO2/0-70% N2 is recommended for bulk packages

* Contains only acid-producing strains. y Contains citrate-fermenting bacteria.

(62). Berne (63) states that the optimum mixture for hard cheeses is 75% CO2/25% N2. Oxygen should be kept at a minimum, because keeping oxygen levels may prevent mold growth (71). The minimum partial pressure of O2 may also be affected by the CO2 levels present. Furthermore, recent studies revealed that even residual oxygen levels of less than 0.5% were insufficient to avoid photooxidation when products were exposed to light (15,16,18,19). Gases with high levels of CO2 should be avoided, particularly for sliced and shredded cheeses, in order to reduce packaging collapse around the product during the absorption of CO2 into the product (64). However, Alves et al. (65) and Eliot et al. (66) found that packaging in high CO2 concentrations (i.e., 75-100% CO2) constitutes an interesting alternative for shredded and sliced Mozzarella cheese.

Different O2 and CO2 levels are expected to result in changes in microbiota flora with subsequent differences in aroma development. However, documentation in this area is almost nonexistent (17).

Active packaging concepts, which may be suitable for matured cheeses, include oxygen absorbers, carbon dioxide emitters, and slow release of antimicrobial and/or anti-oxidative compounds from the packaging materials. Additionally, the use of thermo-reversible dyes and inks is increasing, with the purpose of boosting sales. Mortensen et al. (15-18) applied oxygen absorbers when packaging semihard cheeses and found that the absorbers were noticeably efficient in obtaining low levels of oxygen and postponing photooxidation. The advent of anaerobic conditions or sensory changes due to low oxygen levels were not evaluated in the study.

Packaging materials used at present include laminates consisting of polypropylene (PP, oriented/OPP), polyamide (PA/Nylon), polyethylene (PE), or polyethylene tere-phthalate (PET, amorphous/APET or crystalline/CPET). Polyvinylidene chloride, ethyl-vinyl acetate (EVA), or ethylvinyl alcohol (EVOH) may be used to improve gas barrier properties. Furthermore, aluminum foils are applied and polyvinyl chloride (PVC) is still used in some countries for in-store retail packaging. Packaging materials with low light transmission rates in the critical area, 350-520 nm, may reduce photooxidation considerably (24).

Examples of packaging films include:

1 am laquer/12 am aluminum/2 am lacquer (very low gas and water vapor permeability, light protection, block cheese)

20 am coextruded oriented polypropylene/12 am polyethylene terephthalate/50 am polyethylene/ethylvinyl alcohol/polyethylene (low gas permeability)

15 am oriented polyamide/60 am polyethylene (medium gas permeability, low water vapor permeability—for example sliced cheese, non-lactate-fermenting cheeses).

20-25 am coextruded oriented polypropylene/40-60 am polyethylene (high gas permeability, e.g. lactate-fermenting cheeses).

C. Processed Cheeses

Processed cheeses are prepared by blending and heating natural cheese with emulsifiers to produce a homogeneous paste, which is subsequently packaged and cooled. Product shelf life is extensive (67), and the cheese is most often packaged in materials suitable for hot-filling. Processed cheeses are packed as individual portions or in bulk units. For the individual triangular portions, lacquered aluminum is often used (to prevent corrosion). The influence of different packaging materials (tin cans, polystyrene cups, and low-density polyethylene tubs) and storage temperature was evaluated by Goyal and Babu (68) with tin cans offering the best product protection. To facilitate opening of the portions, an opening device (red colored strips of either cellulose film or polyester) is generally used (60). The individual packages are assembled into circular paperboard cartons or plastic containers with lids. Larger volumes are packed in glass jars or plastic cups of (for example) polyethylene terephthalate (PET) or polypropylene (PP), polystyrene/ethylvinyl alcohol (PS/EVOH), or aluminum. Because many of these products are warm-packaged, the packaging materials must withstand high temperatures. Recently, Schar and Bosset (69) reviewed quality changes in packaged, processed cheese.

D. Mold- and Smear-Ripened Cheeses

This category is defined as cheeses in which lactic acid fermentation is combined with other ripening processes. The body is neither cooked nor pressed and may contain intenal mold (36). The category includes internally ripened blue-veined cheeses (e.g., Roquefort, Dana-blu, Stilton, Gorgonzola), surface-ripened white-mold cheeses (e.g., Camembert, Brie), cheeses ripened by red smear bacterial cultures (e.g., Brick, Muenster), and cheeses ripened by both red smear cultures and mold (e.g., Limburger, Taleggio).

In contrast to fresh cheeses, mold cheeses apparently require only limited light protection. Light has no damaging effects on the thick mycelium layer, which also protects the cheeses by reflecting and absorbing the light (36). To the authors' knowledge, light-induced lipid oxidation in mold and smear-ripened cheeses has not yet been reported, which is probably partly ascribable to the rather strong flavor characteristics of these products, which may mask such off-flavors.

Modified atmosphere packaging of mold cheeses is rather complicated because the mold growth must take place at a controlled rate. Packaging at low oxygen levels extends the shelf life, because it slows down respiration of the mold. However, most mold cheeses are sensitive to anaerobic conditions, which leads to decomposition of the mycelium or formation of off-flavors. Hard, blue cheese may be packaged in pure nitrogen in order to prolong shelf life. Carbon dioxide favors the growth of some molds and suppresses the growth of others (48). Hence, CO2 levels should be tailored to the individual products. Piorgiovanni et al. (70) found that 10% CO2/90% N2 resulted in the best sensory scores when packaging Tallegio cheese. Furthermore, the researchers developed mathematical models describing CO2 development when changing selected packaging variables.

The relative humidity within the package is also a critical factor in controlling the growth and survival of the molds, which implies that the packaging material must also provide an adequate water vapor barrier in addition to ensuring optimal gas permeability conditions.

Mold cheeses are primarily packaged by simply wrapping the material around the product. Flow packaging and deep-drawing are applied for blue cheeses, which are less sensitive to mechanical damage during the packaging process. Many of the soft cheeses are further packaged in wooden boxes, which provide mechanical stability and appeal to the consumers. Perforated foil laminates are used, with the perforation adjusted to allow a certain amount of O2 ingress and yet prevent dehydration (36). If the materials are too tight, the fungus mycelium will autolyse. Hence, perforation is adapted to suit the individual cheeses. Spot or strip lamination between the individual layers of the packaging films is applied to obtain a more open film for white mold cheeses.

Cellophane is extensively used because it allows ingress of O2 and sufficient H2O hydration, preventing fungus autolysis. However, cellophane films are rather expensive and substitutes are increasingly being used, for example packaging films based on polypropylene, which are perforated to obtain the required H2O barrier. Pearlized polypropylene

(produced by addition of silicium oxide) may be used to obtain materials, that are perceived as ''softer'' by the consumers.

Packaging of square and triangular cheeses imposes different dead-fold requirements (little resilience) on the materials, which may be met by increased thickness of the kraft paper, by using aluminum foils, and by metalizing the materials. Goat cheeses contain excessive amounts of water, which should disappear during storage. Hot-melt is used when sealing properties are required, whereas paraffin is used when sealing is not required, namely when the product is packaged in cardboard or wooden boxes after having been wrapped in paper or cellulose-based laminates. Blue-veined cheeses seem to be less dependent on the gas permeability of the packaging material; thus, laminates or less open packaging materials are used for blue-veined cheeses (36). Finally, metal cans are used for heat-sterilized Camembert cheeses.

Examples of packaging materials used include:

Cellophane (transparent or white)/32-40 g/m2 kraft paper/paraffin or hot-melt (Camembert)

Microperforated 15 Am aluminum/cheese varnish (Blue cheese)

20 Am polyproylene/37-45 g/m2 grease-proof paper (Muenster)

22 Am cellophane/5 Am wax/8 Am aluminum/2 Am lacquer (white and white/blue mold cheeses)

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